The molecular organization of the plasma membranes of normal and cancer cells are examined by electron diffraction and dark field electron microscopy under physiological conditions, using hydration stages for electron microscopes. Plasma membranes of rat hepatocytes and Reuber hepatoma cells, lymphocytes and lymphocytic leukemic cells as well as various strains of mammary tumor cells are being studied. The phase transition indicating the reorganization of phospholipid molecules are measured as function of temperature. The phase transition temperature ranges vary from membrane to membrane. These data are interpreted in terms of lipid composition, fluidity, tumorgenicity and matastasizing capacity. Lipid bilayers of various compositions are studied for comparison. Electron diffraction of the membranes at the thin edges of the spreaded normal and transformed cells in culture are also being investigated, using high voltage electron microscopes equipped with hydration stages.